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Published in J. Environ. Qual. 34:534-543 (2005).
© ASA, CSSA, SSSA
677 S. Segoe Rd., Madison, WI 53711 USA

TECHNICAL REPORTS

Organic Compounds in the Environment

Assessment of Herbicide Leaching Risk in Two Tropical Soils of Reunion Island (France)

H. Bernarda, P. F. Chabalierb, J. L. Chopartc, B. Legubea and M. Vauclind,*

a Laboratoire Chimie de l'Eau et de l'Environnement, LCEE-UMR 6008 (CNRS, Université de Poitiers), 40 Avenue du Recteur Pineau, 86022 Poitiers cedex, France
b CIRAD, BP 20, 97408 Saint Denis, Messag cedex 9, France
c CIRAD, 7 Chemin de l'IRAT, 97410 Saint Pierre, France
d Laboratoire d'étude des Transferts en Hydrologie et Environnement, LTHE-UMR 5564 (CNRS, INPG, IRD, UJF), BP 53, 38041 Grenoble cedex 9, France

* Corresponding author (lthe{at}hmg.inpg.fr)

Received for publication March 29, 2004.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX
 REFERENCES
 
Application of organic chemicals to a newly irrigated sugarcane (Saccharum officinarum L.) area located in the semiarid western part of Reunion Island has prompted local regulatory agencies to determine their potential to contaminate ground water resources. For that purpose, simple indices known as the ground water ubiquity score (Gustafson index, GUS), the retardation factor (RF), the attenuation factor (AF), and the log-transformed attenuation factor (AFT) were employed to assess the potential leaching of five herbicides in two soil types. The herbicides were alachlor [2-chloro-2',6'-diethyl-N-(methoxy-methy) acetanilide], atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-1,3,5-triazine], diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], 2,4-D [(2,4-dichlorophenoxy) acetic-acid], and triclopyr [((3,5,6-trichloro-2-pyridyl)oxy) acetic-acid]. The soil types were Vertic (BV) and Andepts (BA) Inceptisols, which are present throughout the Saint-Gilles study area on Reunion Island. To calculate the indices, herbicide sorption (Koc) and dissipation (half-life, DT50) properties were determined from controlled batch experiments. Water fluxes below the root zone were estimated by a capacity-based model driven by a rainfall frequency analysis performed on a 13-yr data series. The results show a lower risk of herbicide leaching than in temperate regions due to the tropical conditions of the study area. Higher temperatures and the presence of highly adsorbent soils may explain smaller DT50 and higher Koc values than those reported in literature concerning temperate environments. Based on the RF values, only 2,4-D and triclopyr appear mobile in the BV soil, with all the other herbicides being classified from moderately to very immobile in both soils. The AFT values indicate that the potential leaching of the five herbicides can be considered as unlikely, except during the cyclonic period (about 40 d/yr) when there is a 2.5% probability of recharge rates equal to or higher than 50 mm/d. In that case, atrazine in both soils, 2,4-D and triclopyr in the BV soil, and diuron and alachlor in the BA soil present a high risk of potential contamination of ground water resources.

Abbreviations: AF, attenuation factor • AFT, log-transformed attenuation factor • BA, Andepts Inceptisol • BV, Vertic Inceptisol • CEC, cation exchange capacity • DT50, half-life • GUS, Gustafson index • MET, maximum evapotranspiration • OM, organic matter • RF, retardation factor


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX
 REFERENCES
 
CONTAMINATION OF SOIL and water resources by agricultural chemicals is a worldwide environmental problem. The dependence of agriculture on the use of potentially toxic compounds and the vulnerability of the natural resources to pollution by agrochemicals leached from the soil surface pose a dilemma for agricultural and environmental regulators.

In this respect, extensive research has been conducted in temperate regions assessing the pollution of surface and subsurface waters as well as sediments with pesticides. However, studies dealing with tropical regions are more scarce while agricultural intensification has led to increased use of pesticides (Racke et al., 1997) and the climatic and soil characteristics are quite different. Within that context, Reunion Island presents some unique features. It has a humid tropical climate with world records of daily and yearly rates of precipitation. Although the annual rainfall amount (ranging between 3000 and 5000 Mm3) largely exceeds the water needs for domestic and agricultural uses (estimated at 120 and 70 Mm3, respectively), the spatial distribution is very heterogeneous between the windward (east) and the leeward (west) coasts and between the highlands and the lowlands. Local authorities have been striving to overcome these imbalances by financing some major hydraulic works such as an ongoing project aimed at diverting 65 Mm3/yr of surface water from the humid eastern to the semiarid western part of the island. The objective is to increase the irrigable area on the west coast to 7000 ha, to boost the total sugarcane production by adding 0.35 million Mg/yr to the 1.80 million Mg/yr that is currently produced in the island.

Soils on the island that originated from volcanic materials generally have high clay and silt contents and they contain an aggregated structure with large porosity and hydraulic conductivity values. Combined with high rainfall intensities, larger values of water and leaching fluxes below the root zone than in temperate regions may be expected (Tomasella and Hodnett, 1997; Hodnett and Tomasella, 2002). In addition, increased rates of volatilization, photolysis, and chemical and biological degradation of pesticides can be expected due to higher air and soil temperature with smaller seasonal variations than in temperate regions (Laabs et al., 2002). For instance, an almost twofold reduction in the half-lives of pesticides for a 5 and 10°C increase of temperature was reported by Racke et al. (1997) and Forum for the Co-Ordination of Pesticide Fate Models and Their Use (1997).

For all the aforementioned reasons, application of organic chemicals, especially herbicides, in the newly cultivated area of Reunion has led to growing concern by the public and regulatory agencies about the possible contamination of the ground water resources of the western part of the island and of the downstream lagoon as well.

A number of comprehensive physically based models are available for site-specific evaluations of the pesticide behavior in the root zone [i.e., MOUSE (Steenhuis et al., 1984); PRZM (Carsel et al., 1984); MARTHE (Thiery, 1990); GLEAMS (Davis et al., 1990); HYDRUS-1D (Kool and van Genuchten, 1999); LEACHP (Hutson and Wagenet, 1992); HYDRUS-2D (Simunek et al., 1996); PESTLA (Van den Berg and Boesten, 1998), among others]. Such models usually require knowledge of a large number of climate, soil, crop, and chemical parameters, which are neither available nor likely to be available in a majority of the cases. Therefore, even if they are excellent research approaches they are not very well designed to provide policymakers with decision-making tools that are easily applicable.

As an alternative, more simple screening models and indices have been proposed for assessing the relative potential of various pesticides to leach beyond the crop root zone and possibly reach ground water. Among them are DRASTIC (Aller et al., 1985), LEACH (Laskowski et al., 1982), GUS (Gustafson, 1991), and RF and AF (Rao et al., 1985). In contrast to indices that include parameters related only to hydrogeological setting (i.e., DRASTIC) or only to chemical properties (i.e., GUS), the AF relies on parameters that characterize pesticide sorption and degradation, key soil properties, and water recharge at some compliance depth. In the past 15 years, it has seen important use and development both in temperate and tropical environments for comparing, in terms of likelihood, the approximate leaching potential of various chemicals (herbicides, insecticides, pesticides) at a given location or for ranking the vulnerability to leaching of different locations with variable soil properties (i.e., Loague et al., 1989, 1990; Giambelluca et al., 1996; Li et al., 1998; Freissinet et al., 1999; Diaz-Diaz et al., 1999). Data requirements for AF are less than those needed for dynamic models, which describe processes over time. However, Rao et al. (1985) and Kleveno et al. (1992) reported a suite of favorable comparisons between AF index and the process-based models CMLS (Nofziger and Hornsby, 1985) for Florida and PRZM (Carsel et al., 1984) for Hawaii, respectively.

The purpose of the work reported in this paper is not to present a new model but to assess by the AF scheme the potential leaching of five herbicides currently used in Reunion under irrigated sugarcane for two soil types of the newly cultivated area. For the calculations of AF, herbicide properties were determined by controlled batch experiments and water fluxes through the vadose zone were estimated by a simple water-balance approach driven by a rainfall frequency analysis and potential evapotranspiration.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX
 REFERENCES
 
Attenuation Factor
Following Rao et al. (1985) and many others, AF is defined as:

[1]
where d (cm) is a reference depth, {theta}FC (cm3/cm3) is the volumetric water content at field capacity, q (mm/d) is the soil volumetric water flux, and DT50 (d) is the herbicide half-life in soil. The term RF (unitless) is the retardation factor, which is defined for a nonvolatile product as:

[2]
where {rho}d (g/cm3) is the soil dry bulk density, foc (%) is the mass fraction of organic carbon, and Koc (cm3/g) is the sorption coefficient. The RF index indicates the relative mobility for a pesticide leaching through soils because of sorption and partitioning between the solid and liquid phases. It takes on values ≥1.0 with higher values indicating less mobility.

It can be shown that Eq. [1] and [2] can be inferred from the standard convection–dispersion equation, assuming that any nonvolatile chemical product undergoes linear, reversible equilibrium adsorption and first-order biochemical decay, while moving downward to soil by leaching at uniform convective flux, the hydrodynamic dispersion being neglected. Intuitively, AF represents the fraction of the initial mass of herbicide applied at the soil surface, which will leach to a prescribed depth in the soil profile. For making relative assessments of herbicide mobility with RF and AF, we have adopted the classification proposed by Liang and Khan (unpublished data, 1987) and Khan and Liang (1988) (Table 1). In addition, because of possible very small and skewed values of AF, we considered, as suggested by Li et al. (1998), the AFT index defined as:

[3]


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  		Table 1. Scales of herbicide mobility and potential leaching as proposed by Liang and Khan (unpublished data, 1987). The numerical values assigned to the various classes are arbitrary; therefore, the classes only indicate relative retardation (RF), attenuation factor (AF), and log-transformed attenuation factor (AFT).

 
We present below the data that are required to calculate the RF and AF indices.

Soils
The study was conducted on a newly irrigated area located in the vicinity of Saint-Gilles community (Fig. 1) . Based on a previous pedologic and geomorphologic survey performed by Raunet (1981) two soil types were selected because of their spatial representativeness. Physicochemical properties determined on soil samples taken from the 0- to 25-cm depth increment are given in Table 2. Vertic Inceptisol (BV) is generally present on terraces located at the bottom of slopes. The sampling area, formerly covered by dry savanna fallow, was irrigated and cropped with sugarcane only a year before the study. The soil has a clayey texture with relatively high values of organic matter (OM) and cation exchange capacity (CEC). Andepts Inceptisol (BA) is commonly encountered on hillslopes above 450 m of elevation. Samples were taken from a field previously cultivated with rain-fed sugarcane, newly irrigated, and cropped with vegetables and sugarcane in rotation. The soil has a loamy-clay texture and has smaller values of OM, CEC, fraction of organic carbon (foc), and pH than BV.



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  		Fig. 1. Map of Reunion Island and location of the study area.

 

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  		Table 2. Selected physicochemical properties{dagger} of the two soils.

 
Herbicides
On the basis of the custom import registry, five herbicides most commonly used on sugarcane in Reunion were selected for the study. They belong to different chemical families. Three are nonionic (i.e., atrazine, alachlor, and diuron) and two are ionic (i.e., 2,4-D and triclopyr).

Parameters describing sorption (Koc) and degradation (DT50) processes of the herbicides were determined through standard batch experiments, performed at 37°C, which represents an average value between air and soil surface temperature at the time of the field application of the products. Results of preliminary kinetics studies showed that equilibrium between sorbed (S) and liquid (C) concentration was reached after about 24 h of reaction (Bernard, 2004). Samples of 20 g/50 mL were used and the isotherms were constructed by varying initial concentration (C0) from 3 to 15 mg/L. Values for Kd and Koc were calculated from Eq. [4]:

[4]
where Kd = focKoc is the partition coefficient (cm3/g).

Degradation experiments were performed on 50-g soil samples. The moisture content was initially set at the field capacity. It was controlled and readjusted once a week. Residual concentrations both in the aqueous and the solid phases (after water or methanol extraction) were measured regularly for a total period of one week to two months depending on the herbicide. Using a first-order kinetics approximation, half-life of the herbicides was calculated by the following equation:

[5]
where k is the degradation rate (d–1) obtained from the linear correlation between Ln(C/C0) and time.

Analytical Procedures
The analysis of herbicides was performed using high performance liquid chromatography (HPLC) equipped with a Model 600E pump, Model 996 photo diode array detector, Model 817 auto sampler injector of 40 µL (Waters, Milford, MA), and a ABZ+ (15-cm length x 4.6-mm i.d. x 3-µm particle size) column and precolumn (Supelco, Bellefonte, PA). The mobile phase was eluted at a flow rate of 0.8 mL/min. The eluting solvents were made of water and methanol (50:50) for atrazine and diuron, of water and acetronitrile (50:50) for alachlor, and of water and acetronitrile acidified at pH 2 with trifluoroacetic acid (0.8{per thousand}) for triclopyr and 2,4-D.

Recharge Estimate
The volumetric water flux through the soil profile (q) was estimated by a water-balance approach driven by a rainfall frequency analysis performed on a 13-yr sample (1989–2001) of daily values measured at one station located in the middle of the newly irrigated area (Fig. 1). The capacity-based model that was used is similar in its principles to the one developed by Chopart and Vauclin (1990) and evaluated against field data. It has been adapted for sugarcane irrigation practices and scheduling. Details can be found in Mézino et al. (2003). The corresponding software package, named SIMULIRRIG, is available from the authors on request (chopart{at}cirad.fr). Herein, only the main features are highlighted.

The model is based on the soil water-balance equation written as:

[6]
where {Delta}S is the daily variation of the soil water storage (mm/d) held between surface and a compliance depth d (cm); R and I are the amounts of precipitation and irrigation, respectively (mm/d); MET is the maximum evapotranspiration (mm/d), and E represents the excess of water in the soil profile leading to either drainage at depth d and/or runoff (mm/d). The term MET is calculated as:

[7]
where Kc (unitless) is the crop factor varying from 0.4 to 1.2 according to the phenological stages of sugarcane (Allen et al., 1998) and PET (mm/d) is the Penman–Monteith potential evaporation.

Since data records of irrigation were not sufficiently detailed in the newly irrigated area to be directly used in the model, the irrigation schedule (i.e., number of events during a 1-yr cycle with a minimum frequency of 4 d, amount of required water per event) was determined on a daily basis by computing from Eq. [6] the amount of water needed to maintain sugarcane at MET and to minimize percolation below the root zone. Those calculations were based on precipitation (R) of the day and on the actual soil water storage {Delta}S. As soon as it is less than 50% of its maximum value (MAWS), the amount of water irrigation required to reach 90% of MAWS is calculated.

Since the soils were found to be reasonably well permeable, runoff was considered to be nil in Eq. [6], and consequently E was assimilated to the water flux q. As long as {Delta}S is smaller than MAWS, q is set at 0. As soon as {Delta}S exceeds MAWS, Eq. [6] with {Delta}S = MAWS gives q. All the calculations were made with d = 60 cm and the corresponding MAWS was estimated at 100 mm of water for both soils.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 APPENDIX
 REFERENCES
 
Herbicide Characteristics
Figure 2 shows adsorption results of the five herbicides on both soils. All isotherms were found to be adequately characterized by a linear equation within the concentration range we used. The corresponding values of Kd and Koc are given in Table 3. For both soils, it appears that the Kd of all the herbicides follows the same ranking as commonly reported for temperate regions (i.e., Weber, 1994; Institut National de la Recherche Agronomique, 2002):



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  		Fig. 2. Adsorption isotherms of herbicides measured in the Vertic (BV) and Andepts (BA) Inceptisols at 37°C. Lines represent fitted linear equations and R2 values are the corresponding coefficients of determination.

 

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  		Table 3. Chemodynamic properties of the five herbicides measured in both soils at 37°C.{dagger}

 
The results confirm that organic matter plays a key role in adsorption of nonionic herbicides as found by other authors (i.e., Chiou et al., 1983; Chiou, 1990; Green and Karickhoff, 1990; Barriuso and Calvet, 1991; Rutherford et al., 1992). As a matter of fact, in the BA soil, which has the lowest organic mater content (Table 2), atrazine, alachlor, and diuron are less adsorbed than in the BV soil, whereas the reverse trend is observed for the acid herbicides (i.e., triclopyr and 2,4-D). In addition to OM, clay content, pH, and CEC have a significant effect on the sorption of ionized solutes, as also noted by Spurlock and Biggar (1984), which are adsorbed in more negative soil clay complex. When comparing the results between the BV and BA soils, the phenomenon appears to be enhanced as pH and CEC decrease.

Figure 3 shows the results of degradation of the herbicides in both soils. As a first approximation the kinetics closely follow a first-order reaction. Values of the decay rates (k) and of the corresponding half-life (DT50) are reported in Table 3. Relative rankings of the herbicide half-life (DT50) are as follows: diuron (16 d) > atrazine (13.6 d) > trichlopyr (3.9 d) > alachlor (1.3 d) > 2,4-D (1.3 d) for the BV soil, and diuron (31.1 d) > atrazine (8.7 d) > alachlor (7.3 d) > trichlopyr (1.4 d) > 2,4-D (0.5 d) for the BA soil. As can be seen, diuron and alachlor degrade faster in the BV soil than in BA because of its higher organic matter content (Table 2), which leads to stronger adsorption. Degradation in both soils likely occurs via biological mechanisms since hydrolysis was found to have little impact (Bernard, 2004). The degradation of atrazine is faster in the BA than in the BV soil, while adsorption was found to be greater in the latter one (Table 3). Degradation in BV is expected to mainly occur as a result of biological processes since hydrolysis is negligible, in contrast with BA where it was found to be a key factor (Bernard, 2004). Triclopyr and 2,4-D, which are anionic, dissipate more quickly in the BA than in the BV soil because of their very weak adsorption in that soil (Table 3) due to its higher clay content (Table 2).



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  		Fig. 3. Decay curves of herbicides measured in the Vertic (BV) and Andepts (BA) Inceptisols at 37°C. Lines are fitted to first-order reactions and R2 values are the corresponding coefficients of determination.

 
For comparison purposes, Table 4 lists, for the selected herbicides, some "best available literature values" of Koc and DT50 and their range of variation. They were taken from the Wauchope et al. (1992) database and from Li et al. (1998) for triclopyr. It can be observed that the measured Koc values are high, but of the same order of magnitude as in the literature, and often fall well within their expected range of variation. On the other hand, most of the estimated half-life values are systematically smaller than those reported in literature. It may be attributed to the fact they were measured at a temperature of 37°C instead of usually 20 to 25°C.


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  		Table 4. Comparison of measured values of the sorption coefficients (Koc) and half-lives (DT50) for the five herbicides in both soils with some literature data estimated at a temperature of 20 to 25°C as reviewed by Wauchope et al. (1992).

 
Recharge Flux
The recharge model and its various components previously described are driven by time-dependent precipitation, irrigation, and reference potential evapotranspiration. The simulation made use of a rainfall frequency analysis performed on a 13-yr sample (1989–2001) of daily records. The study area is characterized (Chopart and Mezino, 2002) by a low mean annual precipitation (600 mm) with high interannual variability (CV = 27.5%) and possible occurrence of very heavy daily rain events (up to 245 mm).

A detailed analysis of the whole series was performed on a 10-d scale. The results are given in Fig. 4a , which presents the percentage of days (10-d period) with daily rainfall above different thresholds. For instance, it can be seen that events above 10 mm/d occur between December and 20 April, with a probability ranging from 5 to 13% (with a mean of about 7%) over fifteen 10-d periods within the series. For the other periods, the probability is lower than 5%. There is less than a 3% probability of rainfall intensities higher than 50 mm/d between late December and the end of February. The probability is less than 1% for the remaining periods of the series.



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  		Fig. 4. Frequency analysis of (a) daily rainfall and (b) daily recharge rate performed by 10-d periods on the 13-yr (1989–2001) series. Lines correspond to various thresholds.

 
The values of recharge rates (q) calculated by the SIMULIRRIG code are given in Fig. 4b. They take into account the irrigation amounts required to maintain sugarcane at the maximum evapotranspiration (MET) during the first 11 months of its 1-yr cycle (the last month being ripening). It can be seen that an excess of water percolating at d = 60 cm is expected only between December and mid-April with the highest values in February and in the first 10 d of March. The number of days with a value of q higher than 10 and 50 mm/d are then expected to occur with probabilities less than 8 and 2.5%, respectively, of these 40 d. Those four decades correspond to the main cyclonic period, which is also the end of the main time of herbicide applications to sugarcane.

Assessment of Herbicide Potential Leaching through the Vadose Zone
The retardation factor (RF), the attenuation factor (AF), and the log-transformed attenuation factor (AFT) were calculated by Eq. [2], [1], and [3], respectively, with the input data given in Tables 2 and 3. The numerical values are presented in Tables 5 (for RF) and 6 (for AFT). Table 6 provides the results for three values of the recharge rate (q = 10, 25, and 50 mm/d). The five herbicides are ranked in terms of their potential to leach beyond d = 60 cm on the basis of the two indices. They were ranked in descending order, a rank of 1 was assigned to the herbicide with the greatest potential, while a rank of 5 indicates the herbicide with the least potential to contaminate ground water. The arbitrary classes of mobility are given in Table 1.


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  		Table 5. Ranking the herbicides based on their mobility in the two soils by the retardation factor (RF) index.

 

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  		Table 6. Ranking of the herbicides based on their potential to leach in both soils at a 60-cm depth by the log-transformed attenuation factor (AFT) index calculated for three recharge rates (q).

 
In addition, the Gustafson index (GUS), defined by:

[8]
was also considered for comparison purposes with other published values (Weber, 1994; Institut National de la Recherche Agronomique, 2002) for temperate regions. Following Gustafson (1991) a pesticide would present a leaching risk for GUS higher than 1.8. The results are given in Table 7. As for the two other indices, the herbicides were ranked in a decreasing order from the highest to the lowest leaching risk.


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  		Table 7. Comparison of calculated values of the Gustafson index (GUS) for the herbicides in the two soils with results published in literature. Herbicides with GUS > 1.8 are considered to present a high potential leaching risk.

 
The following results were found:

Evaluation of Ground Water Pollution Potential
Jury et al. (1987) developed a simple criterion for determining acceptable pesticide environmental fate properties that would result in less than a specified fraction ({epsilon}) of the mass of a product applied at the soil surface that would reach the bottom of the root zone. From Eq. [1] and the physical meaning of AF, this corresponds to the following condition:

[9]
which leads to the inequality:

[10]
with:

[11]

As an example, Fig. 5 presents for both soils the solution of Eq. [10] and [11] calculated with {epsilon} = 0.1%. The solid lines correspond to three scenarios of recharge rates at d = 60 cm. Each of them divides the (Koc – DT50) space into one region of low risk of pollution (to the left) and one region of high risk (to the right). The measured properties of the selected herbicides are represented by symbols. Compounds to the right of the lines are considered to present a ground water risk of contamination at {epsilon}. In the BV soil for atrazine and triclopyr for q ≥ 10 mm/d and for 2,4-D for q ≥ 25 mm/d there is a risk of ground water contamination. Regarding the three other herbicides, the risk of leaching is very low, even for q = 50 mm/d. In the BA soil, atrazine and diuron for q ≥ 25 mm/d, as well as alachlor and triclopyr for q ≥ 50 mm/d present a risk of ground water contamination. On the other hand, the risk is low for 2,4-D even for a recharge rate higher than 50 mm/d. It should be mentioned that the graph contains the same information as Table 6, but it has the advantage of allowing a preliminary assessment of pollution potential of new compounds in various scenarios merely by inserting their measured properties in the (Koc – DT50) plane.



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  		Fig. 5. Phase plot of the herbicides leaching in the two soils at three different fluxes (q = 10, 25, and 50 mm/d). Compounds lying to the right of the lines are considered to present a ground water risk at {epsilon} = 0.1%. DT50, half-life, Koc, sorption coefficient; q, soil volumetric water flux.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
APPENDIX
 REFERENCES
 
Several simple indices were employed to assess the potential leaching of five herbicides in two soil types of Reunion Island. The results presented illustrate the significant and possible controversial role that soil and environmental factors play in determining whether an organic chemical will present a risk to leach through the vadose zone and to contaminate ground water after its application to the soil surface. Within the context of the tropical conditions of the study area, all five herbicides in both soils present a lower risk of leaching than has been reported in literature addressing more temperate regions, due to higher temperature and the presence of highly adsorbent soils. Comparing rankings by the retardation factor (RF) and the log-transformed attenuation factor (AFT) indices allowed for an evaluation of the need to consider losses by dissipation. For instance, herbicides such as 2,4-D and triclopyr received the highest ranking (greatest potential leaching) by the RF scheme for both soils, but because of their short half-lives, they were ranked lower by the AFT index in the Vertic Inceptisols soil (BV) and much lower in the Andepts Inceptisols soil (BA).

The frequency rainfall analysis performed on a 13-yr series coupled with a simple capacity-based water-balance model allowed for studying the influence of water flow through the vadose zone. Values of AFT indicated that the potential leaching of the five herbicides can generally be considered as unlikely for both soils except during the raining period, especially between February and beginning of March where there is a 2.5% level of probability (1 d/yr) of fluxes equal to or higher than 50 mm/d. In that situation, atrazine in both soils, and 2,4-D and triclopyr in the BV soil, as well as diuron and alachlor in the BA soil present a high risk of ground water contamination.

All the results illustrate that climatologic data as well as site-specific soil characterization data are needed for ground water vulnerability assessments. However, it should reemphasized that the index-based approach is not designed to be a predictive tool, but rather a simple method for ranking a number of pesticides in terms of their relative potential to percolate below the crop root zone. Nevertheless, the screening approach is based on several assumptions that would require deeper critical analysis. Among them are the following:

A detailed field experiment is currently in progress with the objectives: (i) to know to what extent the above assumptions are sound, (ii) to evaluate the index approach against a more physically based model and data, and (iii) to estimate the associated uncertainties. Meeting these objectives is necessary before policymakers can be provided with a decision-making tool for screening. The results of this study will be reported in a forthcoming paper.


   APPENDIX
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX
REFERENCES
 
Symbols
C, herbicide concentration in liquid phase; d, reference depth; foc, fraction of organic carbon; k, dissipation rate; Kc , crop factor; Kd, coefficient of partition; Koc, sorption coefficient; q, soil volumetric water flux; S, herbicide concentration in solid phase; {epsilon}, fraction of mass herbicide reaching the bottom of the root zone; {rho}d, soil dry bulk density; {theta}FC, soil volumetric water content at field capacity; {Delta}S, variation of soil water storage.


   ACKNOWLEDGMENTS
 
This research was partially funded by the "Conseil Général de La Réunion." H. Bernard is very grateful to the "Conseil Régional de La Réunion" for providing him with his doctoral fellowship.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX
 REFERENCES
 


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